One of the impacts of climate change is extreme weather events that often bring heavy rainfall and flooding to some areas. Heavy rainfall increases organic runoff into freshwater and coastal waters, and according to a study that was recently published in Scientific Reports, may inhibit the sun's ability to penetrate these waterbodies. As UV light is able to kill pathogens, the sun's ultraviolet (UV) rays provide important ecosystem services, ridding rivers, lakes and coastal waters of pathogens. If this ability is diminished, there is a greater likelihood of waterborne pathogens becoming more prolific.

Research has shown that globally aquatic systems are becoming browner due to the increase in organic material being washed into them from surrounding terrestrial systems — a phenomenon known as "browning". Using a model developed by the National Center for Atmospheric Research (NCAR), this latest study is the first to quantify the impact that dissolved organic material has on limiting the sun's UV rays from disinfecting waterbodies and killing pathogens that lurk in them.

This not only poses a potential health risk for people who are exposed to pathogens when using waterbodies for recreation, but also poses a potential drinking water safety risk — even if the water has been treated. According to Craig Williamson, an ecologist at Miami University and lead author of the paper, dissolved organic matter doesn't only inhibit the ability of the sun to disinfect water, it also renders the water treatment process less effective. Considering that every year between 12-19 million people already fall ill in the US alone due to exposure to waterborne pathogens, this will in all likelihood cause that figure to rise.

For the study, the researchers analyzed water samples collected from lakes in the US and other countries to determine the level of dissolved organic matter present in each of the samples and the wavelengths of ultraviolet light absorbed by the organic matter present.

Then using the Tropospheric Ultraviolet-Visible model — which simulates how UV light is scattered and absorbed as it passes through Earth's atmosphere — the scientists estimated how much of the sun's UV rays reaches the surface of these lakes at different times of the year. They also assessed the amount of reflection and refraction of light from the surface of each lake to determine how much UV light penetrates together with the depth it reaches.

According to the report, "the Tropospheric Ultraviolet-Visible model also calculates the expected disinfecting power of UV light in a particular body of water based on its dissolved organic matter and other characteristics, a measurement known as 'solar inactivation potential (SIP)'. In some cases, researchers calculated the SIP across different parts of, or for different time periods in, the same lake."

From there the researchers were able to quantify the impact that dissolved organic matter had on water quality of lakes, as well as drinking water supplies. For example, modeling of water samples collected prior to and following a severe storm from a site on Lake Michigan — a source of drinking water for over 10 million consumers — showed a 22% reduction in SIP due to the additional dissolved organic matter that flowed into the waterbody from just this one storm.

"Water clarity is dropping in many regions due to factors such as browning, and this research demonstrates that this change is likely decreasing natural disinfection of potentially harmful pathogens," said Kevin Rose, a freshwater ecologist at Rensselaer Polytechnic Institute and coauthor of the paper.